1. Field of the Invention
The present invention relates generally to a digital video signal encoding system and method, and more specifically to such an encoding system and method for encoding a fading video signal made up of a series of video images. Further, the present invention features techniques for detecting fade transitions in digital video images.
2. Description of the Related Art
In order to attain high efficiency of motion picture encoding, it is known in the art to use motion compensation inter-frame prediction that is typically given in the standard proposed by ISO/IEC-13818 (generally known as MPEG-2). According to the encoding technique using the motion compensation inter-frame prediction, an image with which encoding has been completed is used as a reference image which is in turn used to predict the next image. More specifically, the displacements in a currently processing image with respect to the reference image are detected for generating motion vectors. The motion picture inter-frame prediction is then implemented using the motion vectors to generate the predicted image. Subsequently, the information to be used for image prediction is encoded which information includes the motion vectors, motion compensation prediction errors, etc. The prediction errors is subject to a DCT (discrete cosine transform) and then is quantized, wherein the quantizing levels are variable length encoded.
Throughout the instant disclosure, term xe2x80x9cimagexe2x80x9d is interchangeably used with term xe2x80x9cpicturexe2x80x9d.
However, with the above-mentioned conventional encoding techniques, when the motion compensation inter-frame prediction is implemented, the inter-frame prediction errors undesirably become large. This is because the fading images, such as fade-in and fade-out images, exhibits a considerably large time dependent change with respect to the brightness and inter-frame differences between subsequent images.
Therefore, in order to implement the motion picture encoding at a predetermined bit rate, it is necessary to increase the quantizing step sizes and keep the amount of the codes thus generated within a predetermined range. This constant leads to the problem, which causes considerable degradation of picture quality at the time of fading.
In order to overcome the just-mentioned problem, it is proposed in Japanese Laid-open Patent Application No. 08-065565 (conventional art 1), to use the control signal of a video camera for controlling the encoding process. The control signal includes information generated when the fading is carried out. That is, when the fading is commenced, the arrangement of picture types is changed so as to increase bi-directional predicting pictures.
Another approach to overcoming the degradation of picture quality during fade is disclosed in a paper (conventional art 2) entitled xe2x80x9cMotion vector detection during fade in and fade outxe2x80x9d by Jyoji Naitou, et al., pages 275-276, released at the Picture Coding Symposium of Japan held on October, 1992 in Japan. This paper disclosed that the fade transition is ascertained when an average ratio of inter-frame luminance over a series of images exceeds a predetermined value and, at the same time, when a ratio of the average ratio of inter-frame luminance falls within a predetermined range, further, the paper taught that, when detecting the motion vectors for use in implementing motion compensated-frame prediction, the accurate motion vectors can be obtained by compensating for the differences between a reference and current images, whereby prediction errors are able to be decreased.
The above-mentioned conventional techniques however, have encountered the following problems.
The first conventional technique is unable to sufficiently lower the degradation of picture quality. That is, as in this conventional technique, if the picture type arranging structure is changed during fade transitions and if the number of bi-directional prediction pictures is increased, the luminance value of the predicted picture (image) becomes a mid value of the preceding and following core pictures. A cure picture means a reference image, which is used when implementing the inter-frame prediction. However, the luminance value does not necessarily take the value reflecting a ratio of time duration with respect to the reference picture and accordingly, it is not seldom to be unable to properly implement the inter-frame prediction. In connection with the pictures obtained using bi-directional prediction, the inter-frame prediction is performed by making a reference to the preceding and following pictures.
Further, the second conventional technique has encountered the problem that the encoder under ably becomes bulky in terms at configuration. More specifically, with the conventional techniques, the brightness compensation is implemented when detecting the motion vectors. This leads to the fact that three additional devices become necessary: the first device is to calculate an average brightness over an entire image; me second device is to generate an image whose brightness has been compensated: and the third device is to store the resultant image.
Still further, the second conventional techniques is unable to detect fade transitions with high confidence in that each of them carries out the fade detection using only time dependent brightness change. However, in the case of fade-to-white and fade-from-white image transition sequences. The brightness does not change in a simple manner. Therefore, each of the conventional techniques is unable to comply with such case.
It is therefore an object of the present invention to provide an encoder which is able to overcome the above-mentioned problems inherent in the conventional encoding techniques.
Another object of the present invention is to provide a method which is able to overcome the above-mentioned problems inherent in the conventional encoding techniques.
Still another object of the present invention is to provide techniques wherein fade-out transitions of images are encoded by allocating relatively large number of bits at the beginning of the fade-out transitions, and then the number of bits are gradually decreased toward the and of the fade-out.
Still another object of the present invention is to provide techniques wherein the images during fade in transitions are encoded by allocating relatively small number of bite at the beginning of the fade in transitions, and then the number of bits are gradually increased toward the end of the fade in.
These objects are fulfilled by techniques wherein video images including a fade transition are encoded using inter-frame prediction. A fade detector provided to detect a fade transition by way of analyzing incoming images successively applied thereto. The fade detector generates first information indicating if the fade transition is fade-in or fade-out, and also predicts fade duration and then generates second information indicating the predicted fade duration. A bit amount adjuster is provided to adjust allocated bit amount during fade using the first and second information. A motion image encoder is provided to encode the incoming images using the number of bits which has been determined by the bit amount adjuster.
One aspect of the present invention resides in an apparatus for encoding video images including a fade transition using inter-frame prediction, comprising: a fade detector for detecting a fade transition by analyzing incoming images successively applied thereto, the fade detector generating first information indicating if the fade transition is fade-in or fade-out, the fade detector predicting fade duration and generating second information indicating predicted fade duration; a bit amount adjuster for adjusting allocated bit amount during fade using the first and second information; and a motion image encoder for encoding the incoming images using an amount of bits determined by the bit amount adjuster.